Plasma-based light sources, such as laser-produced plasma (LPP) sources, can be used to generate soft X-ray, extreme ultraviolet (EUV), and/or vacuum ultraviolet (VUV) light for applications such as defect inspection, photolithography, or metrology. In overview, in these plasma light sources, light having the desired wavelength is emitted by plasma formed from a target material having an appropriate line-emitting or band-emitting element, such as Xenon, Tin, Lithium or others. For example, in an LPP source, a target material is irradiated by an excitation source, such as a pulsed laser beam, to produce plasma.
In one arrangement, the target material can be coated on the surface of a drum. After a pulse irradiates a small area of target material at an irradiation site, the drum, which is rotating and/or axially translating, presents a new area of target material to the irradiation site. Each irradiation pulse produces a crater in the layer of target material. These craters can be refilled with a replenishment system to provide a target material delivery system that can, in theory, present target material to the irradiation site indefinitely. Typically, the laser is focused to a focal spot that is less than about 100 μm in diameter. It is desirable that the target material be delivered to the focal spot with relatively high accuracy in order to maintain a stable optical source position.
In some applications, Xenon (e.g., in the form of a layer of Xenon ice formed on the surface of a drum) can offer certain advantages when used as a target material. For example, a Xenon target material irradiated by a 1 μm drive laser can be used to produce a relatively bright source of EUV light that is particularly suitable for use in a metrology tool or a mask/pellicle inspection tool. Xenon is relatively expensive. For this reason, it is desirable to reduce the amount of Xenon used, and in particular to reduce the amount of Xenon that is dumped into the vacuum chamber, such as Xenon lost due to evaporation or Xenon that is scraped from the drum to produce a uniform target material layer. This excess Xenon absorbs the EUV light and lowers the delivered brightness to the system.
For these sources, the light emanating from the plasma is often collected via a reflective optic, such as a collector optic (e.g., a near-normal incidence or grazing incidence mirror). The collector optic directs, and in some cases focuses, the collected light along an optical path to an intermediate location where the light is then used by a downstream tool, such as a lithography tool (i.e., stepper/scanner), a metrology tool or a mask/pellicle inspection tool.
For these light sources, an ultra-clean, vacuum environment is desired for the LPP chamber to reduce fouling of optics and other components and to increase the transmission of light (e.g., EUV light) from the plasma to the collector optic and then onward to the intermediate location. During operation of the plasma-based illumination system, contaminants including particulates (e.g., metal) and hydrocarbons or organics, such as offgas from grease can be emitted from various sources including, but not limited to, a target-forming structure and the mechanical components which rotate, translate and/or stabilize the structure. These contaminants can sometimes reach and cause photo-contamination-induced damage to the reflective optic, or damage/degrade the performance of other components, such as a laser input window or diagnostic filters/detectors/optics. In addition, if a gas bearing is used, the bearing gas, such as air, if released into the LPP chamber, can absorb EUV light, lowering EUV light source output.
With the above in mind, Applicants disclose a laser produced plasma light source having a target material coated on a cylindrically-symmetric element and corresponding methods of use.